Level flight in MS FSX

Hello

I've just started with MS FSX and am working through the tutorial lessons.

I'm finding that the aircraft will not fly straight and level. Even on full power, if I let go of the joystick then the aircraft will settle at an angle downwards, losing altitude. I constantly have to pull slightly back on the stick to maintain altitude.

Have I missed something obvious?

Thanks

Mart.

Try applying some nose-up trim?